Thick fog exacerbated by air pollution caused disruption on Thursday in eastern areas of China, as concern grows over the public health implications of the hazardous atmosphere.

Cities cloacked in smog

Forty flights arriving at Shanghai's Pudong International airport were diverted and 20 others cancelled early in the morning amid the fog.

Shanghai’s meteorology authority issued this winter’s first orange fog alarm past midnight last night. Air pollution monitoring sites showed an average PM2.5 reading of the city had soared above 250, a level labelled as “very unhealthy”, throughout Thursday morning.

Photos circulated on social networking websites showed greatly reduced visibility in Shanghai, falling to 50 metres in some areas. Meteorology officials on Wednesday forecasted the weather conditions would continue until a cold air would blow away pollutants next Monday.

Planes were grounded for several hours at one point and half a dozen highways were closed at one point. They were slowly opened before noon.

Classes were suspended on Thursday at kindergartens, primary and middle schools in Nanjing, after the Jiangsu's provincial capital had recorded the worst air pollution of the year on Wednesday, Xinhua reported.

Nanjing education authorities still required teachers to work normally in case parents could not look after their children at home.

The suspension came after the amount of PM 2.5 particles in the air had exceeded the level of 300 on the Air Quality Index (AQI) for 12 hours on Wednesday. The pollution prompted the municipal government command centre overseeing the environent to issue an immediate red alert, the first one this year.

The AQI rose to 377 on Wednesday, according to the Nanjing Municipal Bureau of Environmental Protection's monitoring centre.

Air is rated 'seriously polluted' when AQI reaches 300.

The AQI was measured at 337 at 9am on Thursday. The local weather forecasting department said the pollution could last untill Monday because of lingering low pressure.

Some 25 regions had been hit with smog on Wednesday evening, according to National Meteorological Centre.

The centre said visibility in most parts of Jiangsu and parts of Sichuan, Guizhou, Yunnan, Anhui, Shandong, Fujian and Xinjiang had sunk below 500 metres on Wednesday night and Thursday morning.

Vehicle emissions, the burning of coal for heating and persistent low pressure are blamed for the poor air quality.

In Qingdao, Shandong province, authorities recorded the worst air pollution of 2013 with the AQI reaching 332 on Wednesday. Authorities were forced to take remedial measures, including ordering non-essential government vehicles off the road.

While many parts of the country have had to come to terms with the foul air and consequent inconvenience, a few Nanjing residents have found something to cheer them up.

Residents living in the city’s Gulou district woke up on Wednesday to the rare phenomenon of a “double sun”. It was in fact an optical illusion produced by the smog that made it appear as if a bigger sun was lying on top of a smaller one.

Local scientists said that the smaller sun was simply a reflection caused by the combination of particulates and moisture in the air - further evidence of the damaging pollution.

Meanwhile, Chinese news portal qq.com has put together a photo gallery comparing pictures taken on polluted days in China to those taken in London in the 1950s.